The present invention relates to high-cleanness steel in which the size of inclusion is controlled by an extremum statistical process, and to a toroidal type continuously variable transmission including an input disk, an output disk, a power roller and a cam disk all using the high-cleanness steel.
Speed change gears have mainly been used as conventional transmissions for vehicles such as automobiles. The speed change gears comprises a plurality of gears, and the engagement mode of the gears is varied to transmit torque from an input shaft to an output shaft. However, in the conventional speed change gears, torque is varied step-wise and discontinuously at the time of changing the speed. Thus, the speed change gears have drawbacks such as a loss in power transmission and vibration at the time of changing the speed.
Under the circumstances, a continuously variable transmission, in which torque is not varied stepwise or discontinuously at the time of changing the speed, has recently been put to practical use. In the continuously variable transmission, no vibration occurs at the time of changing the speed, and the loss in power transmission is less than that in the speed change gears. In addition, the continuously variable transmission is fuel-efficient when it is mounted in the vehicle. As an example of the continuously variable transmission, a belt type continuously variable transmission is mounted in some type of passenger cars.
On the other hand, as an another example of the continuously variable transmission, a toroidal type continuously variable transmission has been proposed. The toroidal type continuously variable transmission comprises an input shaft rotated by a drive source such as an engine, an input disk, an output disk, a power roller and a compression device. The input disk is supported on the input shaft and rotated in interlock with the input shaft. The output disk is supported on the input shaft so as to be opposed to the input disk and is rotated in interlock with the output shaft. The power roller is provided swingably between the input disk and the output disk and is rotatably put in contact with both disks. The compression device has a cam disk supported on the input shaft and urges at least one of the input and output disks in such a direction as to approach the input and output disks to each other.
As compared to the belt type continuously variable transmission, the toroidal type continuously variable transmission can transmit a higher torque. It is thus considered that the toroidal type continuously variable transmission is efficient as a continuously variable transmission for use in middle- and large-sized vehicles.
However, transmission of a higher torque is required in the toroidal type continuously variable transmission. Thus, compared to general mechanical components such as gears and bearings to which repeated stress is applied, much higher repeated bending stress and repeated shearing stress is applied to the input and output disks, power roller and cam disk. Accordingly, these disks, power roller and cam disk need to have higher durability than general mechanical components.
Some methods have been proposed to enhance the durability of the input/output disks, power roller and cam disk. For example, Jpn. Pat. Appln. KOKAI No. 7-208568 describes a method of subjecting to predetermined carburizing/carbonitriding the disks and power roller which are the components of the toroidal type continuously variable transmission. Jpn. Pat. Appln. KOKAI No. 8-338493 describes a method of hardening the disks and power roller. Jpn. Pat. Appln. KOKAI No. 9-79338 describes a method of adding special alloy elements to the disks and power roller. It is proposed by these methods to enhance the durability of the toroidal type continuously variable transmission by increasing the strength of the materials.
Besides, micro-defects and microcracks present in the material are factors of breakage. It is known that the degrees of micro-defects and microcracks greatly influence the strength of material against the repeated bending stress. For example, "Metal Fatigue, Influence of Micro-defects and Inclusion" (YOKENDO, 1993) describes that the fatigue limit .sigma.w of the material to which repeated bending stress is applied is expressed by the following equation (1): ##EQU1## where K:
1.43 (in a case where the surface has defects or cracks) PA2 1.41 (in a case where defects or cracks are present adjacent to the surface) PA2 1.56 (in a case where defects or cracks are present inside) PA2 wherein the test reference area S.sub.0 is an area of one test-piece for microscopic observation, an n-number of test-pieces are subjected to the microscopic observation, and when sizes, .sqroot. area.sub.max, of maximum inclusions obtained by the observation, are arranged in order from a smallest one, a standardization variable y.sub.j relative to a j-th .sqroot. area.sub.max is expressed by EQU y.sub.j =-1n[-1n{j/(n+1)}] PA2 wherein the test reference area S.sub.0 is an area of one test-piece for microscopic observation, an n-number of test-pieces are subjected to the microscopic observation, and when sizes, .sqroot. area.sub.max, of maximum inclusions obtained by the observation, are arranged in order from a smallest one, a standardization variable y.sub.j relative to a j-th .sqroot. area.sub.max is expressed by EQU y.sub.j =-1n[-1n{j/(n+1)}] PA2 wherein at least one of the input disk, the output disk, the power roller and the cam disk is formed of a high-cleanness steel, PA2 characterized in that in the high-cleanness steel a relationship between a size of a maximum inclusion in a test reference area S.sub.0 measured by microscopic observation and a standardization variable y is subjected to an extremum statistical process, and an estimation value of the size, .sqroot. area.sub.max, of a maximum inclusion in an estimation area S=30000 mm.sup.2, which is expressed by a relationship given by the following equation calculated by the extremum statistical process, is 50 .mu.m or less: EQU .sqroot. area.sub.max =ay+b PA2 wherein the test reference area S.sub.0 is an area of one test-piece for microscopic observation, an n-number of test-pieces are subjected to the microscopic observation, and when sizes, .sqroot. area.sub.max, of maximum inclusions obtained by the observation, are arranged in order from a smallest one, a standardization variable y.sub.j relative to a j-th .sqroot. area.sub.max is expressed by EQU y.sub.j =-1n[-1n{j/(n+1)}]
.sigma.w: fatigue limit PA1 Hv: hardness of material (relating to strength of material) PA1 .sqroot. area: root (representing the size and shape of defect or crack) of a projection area on which the defect or crack is projected in a maximum major stress direction. PA1 y: standardization variable, and PA1 a and b: constants. PA1 T: return period, PA1 S: estimation area, and PA1 S.sub.0 : test reference area. PA1 .sqroot. area.sub.max : a root of an area of a maximum inclusion present in the test reference area, PA1 y: a standardization variable, and PA1 a and b: constants, PA1 .sqroot. area.sub.max : a root of an area of a maximum inclusion present in the test reference area, PA1 y: a standardization variable, and PA1 a and b: constants, PA1 an input shaft rotated by a driving source; PA1 an input disk supported on the input shaft; PA1 an output disk supported on the input shaft and opposed to the input disk; PA1 a power roller swingably provided between the input disk and the output disk and rotatably put in contact with both disks; and PA1 a compression device having a cam disk supported on the input shaft, PA1 .sqroot. area.sub.max : a root of an area of a maximum inclusion present in the test reference area, PA1 y: a standardization variable, and PA1 a and b: constants,
According to equation (1), it is preferable that mechanical components of, e.g. the toroidal type continuously variable transmission which is used under severe conditions and receives great repeated bending stress and repeated shearing stress be formed of materials in which the size and distribution of micro-defects and microcracks are controlled.
It is generally known that a main defect of steel with high durability suitable for the input/output disks, power roller and cam disk of the toroidal type continuously variable transmission is an inclusion of non-metal oxide. The inclusion is impurities inevitably mixed in steep while the steel is produced through steps of melting, molding and rolling.
There are known some methods of controlling the inclusion, which conform to the JIS (Japanese Industrial Standard) method (JIS G 0555), ASTM (American Society for Testing and Material) method (ASTM E45).
In particular, as methods of controlling the inclusion in the material suitable for the disks, power roller and cam disk which require less inclusion and high cleanness, for example, Jpn. Pat. Appln. KOKAI Publication No. 3-294435 describes a method of controlling the cleanness of material and the above-mentioned "Metal Fatigue, Influence of Micro-defects and Inclusion" describes an extremum statistical process.
The method in Jpn. Pat. Appln. KOKAI Publication No. 3-294435, the material is re-melted by using electron beams and relatively large inclusion is floated, thereby controlling the cleanness of material.
In the extremum statistical process, the size of greatest inclusion within unit area S.sub.0 of each of a plurality of test-pieces is examined and then a statistical process is performed, thereby estimating the size of greatest inclusion within an area ("estimation area" hereinafter) S which requires estimation.
In the extremum statistical process (see "Metal Fatigue, Influence of Micro-defects and Inclusion", Keigi MURAKAMI, Yokendo, Mar. 8, 1993, 1st ed. pp. 233-261), the size of greatest inclusion is estimated through the following steps.
At first, a plane in a test-piece, which is perpendicular to a major stress direction, is cut out. A surface of the cut-out plane (hereinafter referred to as "to-be-tested surface") is polished and mirror-finished. As the above-mentioned unit area S.sub.0, a test reference area substantially equal to one visual field of an optical microscope or a camera is determined. The test reference area S.sub.0 is an area per test-piece to be observed.
Thereafter, the to-be-tested surface is observed by the optical microscope or camera and a maximum inclusion in the test reference area S.sub.0 is chosen. Root .sqroot. area.sub.max of the area of the maximum inclusion is measured. The measurement of .sqroot. area.sub.max of the maximum inclusion is repeated by n times such that the tested portion does not overlap. Root .sqroot. area.sub.max of the area of the maximum inclusion indicates the size of the maximum inclusion.
The roots .sqroot. area.sub.max of the areas of an n-number of measured maximum inclusions are arranged from the smallest one, and number j (j=1 to n) is added to each root. Using the following equations (2) and (3), cumulative distribution function F.sub.j (%) and standardization variable y.sub.j are calculated. EQU F.sub.j ={j/(n+1)}.times.100 (2) EQU y.sub.j =-1n[-1n{j/(n+1)] (3)
The root .sqroot. area.sub.max is indicated on the abscissa of an extremum probability sheet and F.sub.j and y.sub.j are indicated on the ordinate of the sheet. Data on the inclusions 1 to n is plotted on the extremum probability sheet. A maximum inclusion distribution straight line is calculated with respect to the standardization variable y.sub.j and .sqroot. area.sub.max. The maximum inclusion distribution straight line is expressed by the following equation (4): EQU .sqroot. area.sub.max =a.times.y+b (4)
where
The standardization variable y is expressed by the following equations (5) and (6): EQU y=-1n[-1n{(T-1)/T}] (5) EQU T=(S+S.sub.0)/S.sub.0 (6)
where
The estimation area S for estimating the size of the maximum inclusion is freely set and, using equations (4) to (6), the root .sqroot. area.sub.max of the area of the maximum inclusion is estimated as the size of the maximum inclusion in the estimation area S.
By the above-described method of controlling the inclusion, the cleanness of the steel suitable for the functions of an anti-friction bearing or gears is controlled. However, as compared to a general anti-friction bearing 53 or gear shown in FIG. 15, a repeated stress with a high absolute value is applied to an input/output disk 50 (FIG. 12) and a power roller (FIG. 13), which are components of a single cavity type half toroidal continuously variable transmission or one type of toroidal continuously variable transmission, and to a cam disk 52 (FIG. 14) which is a component of a double cavity type half toroidal continuously variable transmission. For example, a contact pressure of about 4.0 GPa and a bending stress of about 90 kgf/mm.sup.2 are applied to the disk 50 and power roller 51.
The volumes of portions S1, T1, S2 and T2 shown in FIGS. 12 to 14, where relatively large stress is applied, are greater than the volume of the portion U in FIG. 15 where relatively large stress is applied. In the cam disk 52 shown in FIG. 14, a high compressive stress acts on the portion encircled by a dot-and-dash line S2 and a high tensile stress acts on the portion encircled by a two-dot-and-dash line T2.
Under the circumstances, when the disk 50, power roller 51 and cam disk 52 of the toroidal type continuously variable are designed, a plurality of test-pieces of the disk 50, power roller 51 and cam disk 52 are formed. Different repeated stresses are applied to the test-pieces, and the number of repetition of stress until the destruction is found. An S-N graph is prepared by using a repeated stress and the number of repetition of stress.
The fatigue limits .sigma.w of the disk 50, power roller 51 and cam disk 52 are found from the S-N curve in the S-N graph. Then, the fatigue limits .sigma.w are divided by safety factors in the range from, e.g. 1.2 to 2.0. Thus, allowable stress of each of the disk 50, power roller 51 and cam disk 52 is found. The allowable stresses thus obtained are used for design.
Since the method of finding the allowable stress of each of the disk 50, power roller 51 and cam disk 52 by using the S-N graph is performed on a limited number of test-pieces, all defects in the material are not necessarily evaluated. Accordingly, in the method using the S-N graph, sufficient durability may not be obtained. In addition, in this method, the above-mentioned repeated tests have to be conducted each time the specifications of the toroidal type continuously variable transmission are changed. This may increase the time and cost of development of the toroidal type continuously variable transmission.
Steel 54 shown in FIGS. 16A and 16B is used as material of the disk 50, power roller 51 and cam disk 52. The steel 54 is shaped like a rod by rolling, etc. According to the method conforming to the aforementioned JIS method or ASTM method, when the steel 54 is relatively thin, a plane 56 including part of a surface 55 of steel 54 and a center line P1 is used as a to-be-tested plane.
On the other hand, when the steel 54 is relatively thick, as shown in FIG. 16B, a plane 57 extending in the direction of rolling of steel 54 and having a center at a midpoint between the center line P1 and surface 55 is used as a to-be-tested plane. According to the extremum statistical process, too, as described in the aforementioned "Metal Fatigue, Influence of Micro-defects and Inclusion", the to-be-tested plane is determined on the basis of the above-mentioned position.
In general, in the steel 54 formed in a rod by rolling, etc., the inclusion increases toward the central part of the steel 54 and the cleanness decreases. In the steel 54, the inclusion decreases toward the surface 55 and the cleanness increases.
According to the above-mentioned conventional JIS method, ASTM method and extremum statistical process, the portion near the surface 55 having high cleanness is included in the to-be-tested plane, and thus the cleanness of the steel 54 is averaged. As a result, in the conventional JIS method, ASTM method and extremum statistical process, the cleanness of the entire steel 54 used as material may be inexactly determined. That is, in the conventional JIS method, ASTM method and extremum statistical process, the cleanness of the central portion, which is actually low, may be determined to be high. Thus, it cannot be said that the size of the maximum inclusion is exactly estimated.
These methods are unsuitable, in particular, for testing the inclusion in the material of the mechanical components, such as disk 50, power roller 51 and cam disk 52 of the toroidal type continuously variable transmission, where the volumes of portions S1, T1, S2 and T2 receiving repeated stress are large and both central and surface portions receive relatively high repeated stress.
Therefore, in the toroidal type continuously variable transmission formed of steel, the cleanness of which is controlled by the above-described inclusion control method, the allowable stress at the time of strength design is not necessarily ensured.